what momentous event occured in the first year of Cyrus king of Persia?

Answer 1

Assuming you mean Cyrus II 'The Great' who came to throne of Persia in 549BCE, the most momentous event of the 1st year of his reign was his defeat of the Medes in 548BCE. The Medes had hitherto been seen as the strongest nation in that part of the Middle East. It is reported that, contrary to normal practise at the time, he treated the defeated Medes, including their king Astyages, very leniently. Neither did he raze their city, Ecbatana as, again, was usual practise and, indeed, that city became the virtual capital of Cyrus's empire